The present embodiments relate to an x-ray device with a control electrode for controlling a flow of electrons generated between a cathode and an anode.
X-rays are in widespread use in medical diagnosis. In such applications, the x rays may be generated by x-ray tubes. An x-ray tube may include a housing in which a vacuum is established. The x-ray tube also includes an anode and a cathode, which are found inside the vacuum housing. For operation, the cathode may be heated up in order to assist the emission of electrons. The x-rays are then generated by application of a voltage between the anode and the cathode. This may involve a high voltage in the range 40-125 kV, which is provided by a generator. The voltage applied allows electrons to exit from the cathode. The electrons are accelerated and, on striking the anode, generate x-rays that leave the housing through an exit window.
For better control of the irradiation, control electrodes (e.g., a control grid) may be used. Instead of setting up and removing the voltage between the anode and the cathode, the control electrode is arranged in the housing between the anode and the cathode in such a way as to allow the flow of electrons to the anode to be stopped by application of a control voltage between the electrode and the cathode. The application of the control voltage may be a blocking voltage, which may also be generated by the generator. This method is described, for example, in publications DE 101 36 947 A1 and JP 11204289 A.
During operation of powerful grid-blockable tubes with a high switching speed when high voltage is present and steep-edged switching of the radiation (equivalent to applying a load), a collapse or an overshoot in the high voltage is evident. The deviation of this actual tube voltage from the nominal value amounts quantitatively to up to around 40% and leads to a non-monochromatic radiation at the beginning or end of radiation. The deviation may also lead, in the case of an overvoltage peak, to an increased risk of flashovers and to damage caused by these flashovers and other damage. This has a reciprocal effect to the imaging time on the x-ray quality and is thus of significance with very short pulses in particular.